1. Field of the Invention
The present invention relates to a power supply system, an electric vehicle with the power supply system mounted thereon, and a method of charging a storage battery included in the power supply system. More particularly the present invention pertains to a technique of keeping the remaining charge of a storage battery at or above a predetermined level in a power supply system with fuel cells and the storage battery.
2. Description of the Prior Art
A proposed power supply system includes fuel cells and storage batteries as power sources, wherein the fuel cells charge the storage batteries and the storage batteries charged to a sufficient level supply electric power to a loading (for example, JAPANESE PATENT LAYING-OPEN GAZETTE No. 6-124720). This power supply system includes a plurality of storage batteries, one of which is connected to the loading while the other is connected to the fuel cells. The storage battery having a less remaining charge is charged by the fuel cells, while the other storage battery supplies electric power to the loading. This structure ensures the sufficient charge state of the storage battery connected to the loading, thereby stably supplying electric power to the loading, such as a driving motor of the electric vehicle.
Since the proposed power supply system has a plurality of storage batteries, however, a relatively large space is required for installation of the power supply system. Especially when the power supply system is mounted on a vehicle as a power source of a motor for driving the vehicle, the relatively bulky power supply system in the restricted space of the vehicle undesirably reduces the degree of freedom in design of the vehicle. Only one storage battery is connected to the loading at one time. Each of the storage batteries is thus required to have a marginal capacity, in order to ensure sufficient outputs in case of an abrupt increase in loading, for example, at the time of starting the vehicle or going up a slope. The increase in capacity of the storage battery, however, results in increasing the weight. In some cases, it is accordingly impossible to mount a plurality of storage batteries having a sufficient capacity on the vehicle.
An improved structure has been proposed to reduce the size of the conventional power supply system that has a plurality of storage batteries and enables the storage batteries to be switched and alternately supply electric power to the loading. This improved structure has fuel cells and a storage battery, which are connected in parallel and enables at least either one of the fuel cells and the storage battery to supply electric power to the loading. In case that the loading is smaller than a predetermined level and the fuel cells have a marginal output, the fuel cells can charge the storage battery while driving the loading. In case that the loading is greater than a predetermined level, on the other hand, both the fuel cells and the storage battery work to drive the loading. This structure decreases the required capacity of the storage battery, thereby reducing the size of the power supply system.
In the power supply system of this structure that enables reduction in size, however, the characteristics of the fuel cells and the charge state of the storage battery may cause problems at the time of starting the power supply system. The fuel cells produce an electromotive force through electrochemical reactions and thus generally do not attain required outputs or stability at room temperature at the time of a start. Polymer electrolyte fuel cells, for example, realize high-output and stable power generation in the temperature range of 80.degree. C. to 100.degree. C. It is accordingly required to carry out warm up the fuel cells and increase the internal temperature of the fuel cells at the time of a start.
In the power supply system with the fuel cells and the storage battery, in case that the storage battery has a small remaining charge at the time of starting the power supply system, an insufficient output from the storage battery causes the fuel cells to be exposed to a large loading. FIG. 10 is a graph schematically showing the output characteristics representing the relationship between the voltage and the electric current at the time of power generation by the fuel cells. In the process of power generation by the fuel cells that are in the stationary state and thus normally operable, the fuel cells can output high voltages over a relatively wide range of electric current, although the voltage gradually decreases with an increase in electric current. In case of the fuel cells that have not yet reached the stationary state, on the other hand, the voltage abruptly drops with an increase in electric current output from the fuel cells. When the storage battery has an insufficient remaining charge at the time of starting the power supply system, the fuel cells are exposed to an extreme loading, which may cause a voltage drop and damage the function of the fuel cells as the power source.
An excess flow of electric current through the fuel cells in the insufficient warm-up state causes a voltage drop as well as uneven power generation in the respective unit cells constituting the stack of fuel cells, which results in an abnormal phenomenon, such as a change of poles in part of the unit cells. The change of poles is a phenomenon that reverses the anode and cathode in the cell reactions. Such an abnormal phenomenon makes the voltage unstable and causes energy that has not been converted to electrical energy to be released as thermal energy and partially increase the temperature of the fuel cells. The partial temperature increase damages the elements of the fuel cells and shortens the life of the fuel cells.